The present invention relates to combustors for turbines and particularly relates to apparatus and methods for enhancing combustion at non-baseload operation conditions in an otherwise lean premixed gutter-stabilized combustion system.
Generally, two combustion techniques have been used in the past in combustors for turbines. One technique, known as a diffusion flame process, involves injecting fuel into the air flow through the combustor and burning the fuel as it mixes with the air. The advantages of the diffusion flame combustion technique include self-regulation, and flame stability under a wide variety of conditions. Thus, combustors employing the diffusion combustion process are designed to maintain the flame in a specific location and avoid blow-out and movement either upstream or downstream. Diffusion combustion processes, however, produce a very high flame temperature which, in turn, causes undesirable emissions such as high NO.sub.x emissions. With current emphasis on low-pollution, low-emission turbines, one method of reducing the high flame temperature in the diffusion combustion process, and hence the level of pollutants, is to provide air in excess of that necessary for complete combustion. In this process, fuel and air are premixed upstream of the burning zone of the combustor in a significantly lean fuel/air mixture. When the lean premixed fuel and air is introduced into the combustion zone, the mixture ignites and burns, resulting in a flame temperature that is reduced because of the available excess air.
Because the flame temperature is lower in lean premixed combustion systems, the flame is also more unstable than in the diffusion combustion system. To provide flame stability in a lean premixed combustion system, gutters are often employed upstream of the combustion zone to create low velocity, recirculating flow regions and hence hold the flame in these gutter wakes downstream of the gutters. Gutters have thus been used in lean premixed combustion processes to stabilize the burning process, while maintaining cooler flame temperatures with lower emissions.
Turbines are normally operated at baseload conditions in a lean premixed combustion mode at a predetermined fuel/air ratio. In turbines used, for example, for driving a generator and producing electricity, fuel/air ratios vary across the load. Thus, in industrial gas turbines operating at a single speed and at a constant air flow through the combustion system, any load reduction requires a corresponding reduction in fuel. While the turbine operates at the baseload condition for maximum efficiency, there are conditions such as ignition, acceleration of the rotor to operating speed, synchronization of the rotor with the generator, or low-load operation, situations where a non-baseload operating condition exists.
At these non-baseload, typically low-load conditions, where lower fuel/air ratios are used, the premixed lean burning process may potentially become unstable and inefficient and approach or cross the lean flammability limit where blow-out occurs. Consequently, at these non-baseload conditions, it has been found necessary to employ the diffusion combustion process rather than the lean premixed combustion process, because the diffusion combustion process is efficient and stable at low loads. Thus, fuel may be injected directly into the flow, for example, from the gutters or from the hub of the gutters used for the lean premixed operation. However, if the diffusion combustion process is used in a system geometrically designed for lean premixed combustion, the injected fuel mixes out into the excess air necessary to the lean premixed operation, resulting in undesirable emissions including carbon monoxide and unburned hydrocarbons. Consequently, in the evolution of the present invention, it has been found that, while a lean premixed combustor requires excess air at baseload operation, introducing a diffusion burning process into a fixed combustor geometry designed for lean premixed combustion, particularly at low fuel/air ratios, causes the flame to be inefficient and the fuel to be incompletely burned, hence releasing unburned hydrocarbons and carbon monoxide.
According to the present invention, advantage is taken of the geometry of the lean premixed combustor system for stabilizing the flame to produce flow conditions in the combustor to enhance the non-baseload or low-load burning process using the diffusion combustion process. This is accomplished in the present invention by segregating the flow through the combustor into discrete flow fields, each containing only a fraction of the total flow past the gutters which are used for stabilization of the flame in the lean premixed combustion process, and employing the diffusion combustion process in a limited number of the discrete flow fields. This aids ignition and low fuel-to-air ratio operation by creating a locally higher fuel-to-air ratio, enabling hotter and more complete combustion to occur than if all of the air were directly involved in the combustion. The localized hotter burning produces less carbon monoxide and unburned hydrocarbon emissions, while maintaining flame stability. The present invention thus uses the gutters employed in the lean premixed combustion process for flame stabilization to establish the discrete flow fields which afford both flame stability and higher combustion efficiency when using the diffusion combustion process in an otherwise fixed geometry combustor for lean premixed combustion operation.
More particularly, the gutters are arranged to produce counter-rotating concentric flow fields. That is, the gutters are arranged in the usual radial array for lean premixed combustion operation but are turned or canted at different radial locations to provide flow components at circumferentially opposite directions establishing respective discrete flow fields. Consequently, concentric counter-rotating swirling flow fields are provided within the combustion enclosure. These flow fields form an interface or shear layer which isolates the flows from one another. Direct injection of fuel may therefore be provided into a subset of the overall flow field, typically the radially innermost flow field, wherein a diffusion combustion process may be established within the above subset of the total flow field using only a fraction of the total air flow through the combustor. That is, the fuel is directly injected into a zone which has highly turbulent swirling air, and which is isolated from the remainder of the flow through the combustor by a shear layer established between the discrete flow fields.
Additionally, with the inner and outer gutter arrays affording concentric and discrete flow fields, it will be appreciated that one flow field, the radially inner flow field, has a higher concentration of gutter area and, hence, a higher blockage of the flow through the combustor. This permits greater recirculation within the downstream flow field and consequently enhanced flame stability and more time for the fuel introduced in the diffusion process to burn.
In a preferred embodiment according to the present invention, there is provided a combustor for a turbine comprising an enclosure for receiving a flow of lean premixed fuel and air and combustion thereof in a combustion zone for producing low emissions at baseload operation of the turbine, and an array of gutters disposed in the enclosure upstream of the combustion zone, the gutters having an elongated apex and surfaces divergent therefrom extending in a downstream direction in the enclosure for stabilizing the flame in the combustion zone when combusting the premixed fuel and air. The gutters are configured and arranged to isolate the flow through the enclosure downstream of the array of gutters into at least two discrete flow fields, each containing a fraction of the total flow past the gutters. Means are provided for introducing fuel into one or more of the discrete flow fields during turbine operation at non-baseload operating conditions to create in one discrete fluid flow field combustion by a diffusion process with a locally higher fuel-to-air ratio enabling hotter and more complete combustion at non-baseload conditions.
In a further preferred embodiment according to the present invention, there is provided a combustor for a turbine comprising an enclosure for receiving a flow of lean premixed fuel and air and combustion thereof in a combustion zone for producing low emissions at baseload operation of the turbine, means in the enclosure for stabilizing the flame in the combustion zone when combusting the premixed fuel and air and means for isolating the flow through the enclosure into at least two discrete flow fields each containing a fraction of the total flow through the enclosure. Means are provided for introducing fuel into at least one of the discrete flow fields during turbine operation at non-baseload conditions to create in at least one discrete fluid flow fields combustion by a diffusion process with locally higher fuel-to-air ratio enabling hotter and more complete combustion at lower than baseline operating conditions.
In a further preferred embodiment according to the present invention, there is provided a method of operating a combustor for a turbine, comprising the steps of providing a combustor having an enclosure, a combustion zone and an array of gutters upstream of the combustion zone, supplying a flow of lean, premixed fuel and air past the array of gutters for combustion in the combustion zone affording lean premixed low-emission turbine operation at baseload conditions, forming at least two discrete flow fields in the enclosure downstream of the array of gutters, with each flow field containing a fraction of the total flow past the array of gutters, isolating the flow fields one from the other to form an isolated combustion zone and providing fuel into the isolated combustion zone at non-baseload operating conditions to create a locally higher fuel-to-air ratio in the isolated combustion zone, enabling hotter and more complete combustion therein than if all the flow was involved in combustion in the entire combustion zone.
In a further preferred embodiment according to the present invention, there is provided a method of operating a combustor for a turbine, comprising the steps of providing a combustor having an enclosure and a combustion zone, supplying a flow of lean, premixed fuel and air into the combustion zone for combustion therein thereby affording lean premixed low-emission turbine operation at baseload conditions, forming at least two discrete flow fields in the enclosure, with each flow field containing a fraction of the total flow through the enclosure, isolating the flow fields one from the other to form an isolated combustion zone and providing fuel into the isolated combustion zone at non-baseload operating conditions to create a locally higher fuel-to-air ratio in the isolated combustion zone, enabling hotter and more complete combustion therein than if all the flow was involved in combustion in the entire combustion zone.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for facilitating operation of the combustor of a turbine designed for lean premixed gutter stabilized combustion at baseload operating conditions by enhancing the burning process when operating in a diffusion combustion mode at non-baseload conditions employing the geometry of the otherwise lean premixed gutter stabilized combustion system.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.